Year	Percent
1960	48.9
1961	50.001
1962	51.104
1963	52.206
1964	53.307
1965	54.402
1966	55.494
1967	56.581
1968	57.663
1969	58.735
1970	59.8
1971	62.281
1972	64.703
1973	67.046
1974	69.309
1975	71.484
1976	73.566
1977	75.615
1978	77.604
1979	79.475
1980	81.231
1981	82.864
1982	84.384
1983	85.792
1984	87.095
1985	88.291
1986	89.392
1987	90.4
1988	90.4
1989	90.4
1990	90.4
1991	90.4
1992	90.4
1993	90.839
1994	91.261
1995	91.664
1996	92.052
1997	92.422
1998	92.776
1999	93.115
2000	93.44
2001	93.75
2002	93.925
2003	93.975
2004	94.155
2005	94.453
2006	94.736
2007	95.007
2008	95.264
2009	95.508
2010	95.74
2011	95.961
2012	96.171
2013	96.37
2014	96.559
2015	96.739
2016	96.91
2017	97.072